(1) Field of the Invention
This invention relates to articles having coatings thereon to reduce reflection and, if desired, to increase transparency. The invention particularly relates to such single layer coatings as are believed to provide a variable gradient index of refraction at the surface of the articles.
(2) Description of the Prior Art
Various types of coatings to reduce reflectivity and improve the transparency of articles such as lenses and windows, and to improve the efficiency of solar cells and solar absorption panels are well known. Perhaps the best known are the single, or preferably multiple, layer interference coatings used on optical lenses, filters and as antireflecting films used on windows. While such coatings are desirable in that they are durable and are known to provide an extremely low reflectivity at specific wavelengths, they exhibit a number of limitations. For example, the optical characteristics of such single layer films are highly sensitive to the wavelength, such that multiple layer coatings must be employed. However, if such multiple layer coatings are used, a significant sensitivity to the direction of incident light results. It has not heretofore been possible to achieve wavelength independent, wide angle response simultaneously. Furthermore, such films are relatively expensive to produce, requiring careful control of the thickness of the coating as well as multiple coating operations.
In addition to such articles in which the reflectance therefrom is reduced via a coating having optical interference characteristics, it is also known to provide articles in which the reflectance is reduced by providing a microstructured surface over which the effective index of refraction varies continuously from the substrate to the ambient environment. See, for example, U.S. Pat. No. 2,432,484 (Moulton). It is believed that the highly sensitive vision of nocturnal insects such as moths is at least partly due to the low reflectivity from the surface of the eyes due to the presence of such a microstructure on the surface of the eye. (G. G. Bernhard et al, Acta Physiologica Scand., Vol. 63 243, pp. 1-74 (1965). More recently, a technique has been disclosed for reducing surface reflection by applying a layer of photosensitive material to the surface, after which the layer is exposed to a regular pattern to light and developed to convert the regular light pattern into a regular array of protuberances (Clapham and Hutley, U.S. Pat. No. 4,013,465).
While investigators have thus appreciated the desirability of forming such surfaces for antireflective applications, it has not heretofore been known how to provide such surfaces having an acceptable degree of uniformity across an extended surface nor how to produce surfaces in a commercially acceptable manner.
Solar collectors utilizing porous coatings to increase the absorptivity and to minimize the radiation loss due to reverse reflected radiation (visible or IR) are also known. It is also known to utilize micropores, grooves or other "textural" effects in such devices to effect an increase in absorptance. (J. Vac. Sci. Tech., Vol. 12, No. 1, Jan/Feb (1975)). For example, U.S. Pat. No. 3,490,982 (Sauveniere et al) discloses a method of treating a glass surface to provide a microstructured surface exhibiting reduced reflectivity. Such coatings, surface treatments and the like have not proven commercially acceptable, possibly due to the instability of the surfaces, cost or inability to provide uniform surfaces over extended areas.
While not heretofore directed to the formation of articles having desirable optical properties, it is known to treat metal surfaces such as aluminum with water and to thereby form a porous oxide or hydroxide (boehmite) surface layer. See, for example, U.S. Pat. Nos. 3,871,881 and 3,957,197, which patents disclose prior inventions of one of the present inventors, V. Mikelsons.